Energy requirements for unfolding and membrane translocation of precursor proteins during import into mitochondria

ATP is involved in conferring transport competence to numerous
mitochondrial precursor proteins in the cytosol. Unfolded precursor
proteins were found not to require ATP for import into
mitochondria, suggesting a role of ATP in the unfolding of
precursors. Here we report the unexpected finding that a hybrid
protein containing the tightly folded passenger protein
dihydrofolate reductase becomes unfolded and specifically
translocated across the mitochondrial membranes independently of
added ATP. Moreover, interaction of the precursor with the
mitochondrial receptor components does not require ATP. The results
suggest that ATP is not involved in the actual process of unfolding
during membrane translocation of precursors. ATP rather appears to
be necessary for preventing the formation of improper structures of
precursors in the cytosol and for folding of imported polypeptides
on (and release from) chaperone-like molecules in the mitochondrial
matrix.